A thorough review and critical evaluation of all experimental sulfur potential and phase diagram data available from the literature has been made for optimizing the Gibbs energy of the copper-sulfur liquid phase at 1 bar total pressure. The extended modified quasichemical model serves as a basis for the mathematical expression of the Gibbs energy of binary Cu-S solutions over the complete composition range. A structurally versatile molten phase ranging from highly metallic via sulfur-rich to pure sulfuric is described simultaneously by a single Gibbs energy function. In combination with the recently published Gibbs energies of all Cu-S solid phases, the complete T–x phase diagram as well as for the first time the log(p _{S 2}/ bar) - 1 / T diagram is calculated. A limited set of obtained model parameters reproduces a large body of data within experimental uncertainties.